In Long Term Evolution (LTE), the semi-persistent scheduling (SPS) concept for uplink (UL) and downlink (DL) are adopted to decrease the control signaling transmission in the Uu interface.
For DL, only one Physical Downlink Control Channel (PDCCH) control signaling is needed to activate the SPS configuration which is configured by RRC. After that, no PDCCH is needed for each SPS initial transmission since the User Equipment (UE) knows the TF from its previous RRC configuration. On the other hand, for the SPS retransmission, due to the asynchronous DL HARQ, a DL PDCCH is needed to indicate the HARQ process ID so that the UE can perform the HARQ combining correctly. One HARQ process is associated with one HARQ buffer in which data to be re-transmitted is stored for HARQ combining during re-transmission. Typically one HARQ buffer can only store one data block, i.e., the data being transmitted in the course of the HARQ process. In some circumstance, such as a Multiple Input Multiple Output (MIMO) case, a HARQ buffer may store two data blocks. In such a case, the receiver receives two data blocks at the same time within one HARQ process. The two data blocks are stored into respective HARQ buffers so that HARQ combining can be performed.
Since no PDCCH is needed for SPS initial transmission, the UE does not know which HARQ process the current SPS transmission is mapped to and thus ambiguity occurs. For example, the SPS data 1 is transmitted by HARQ process ID1 and HARQ retransmission is needed. Before the SPS data 1's retransmission, another SPS data 2's initial transmission happens in HARQ process ID2 which also needs HARQ retransmission. Then in the following Transmission Time Interval (TTI), the base station sends down a DL PDCCH for retransmission indicating HARQ process ID1. However, at the UE side, the UE does not know whether the HARQ process ID1 is used to carry SPS data 1 or SPS data 2, and thus it is impossible for it to perform the HARQ combining correctly.
There are two solutions proposed against this problem:
1. The first one is to reserve several dedicated HARQ processes for SPS transmission. For example, assuming 3 HARQ processes are reserved for SPS configured by RRC. For the first SPS transmission, HARQ process ID1 is used, and HARQ process ID2 is used for the next SPS transmission and HARQ process ID3 is used for the third SPS transmission, and then HARQ process ID1 for the fourth and so on. The concept is shown in FIG. 1, where 2 HARQ process IDs are reserved as an example. In the first SPS transmission occasion, the HARQ process ID1 is used and the HARQ process ID2 is used for the second one. So the UE can differentiate each retransmission from the HARQ process ID in the DL PDCCH and perform correct HARQ combining. Form this point of view, no ambiguity happens. However, there is no HARQ process ambiguity only for the consecutive SPS data transmission. However, reserving more than one dedicated HARQ process for SPS will decrease throughput for the Dynamic Scheduling (DS) since these HARQ processes can not be used for DS even when no SPS HARQ retransmission is needed, which is shown in FIG. 2. As shown in FIG. 2, the worst case may occur where almost 20% transmission chances are wasted due to the dedicated HARQ process reservation for SPS. This is even worse for VoIP service. During the silence period of the VoIP service, the time interval between two SPS data blocks is about 160 ms. The throughput for DS will be largely affected if the HARQ processes reserved for 160 ms can not be released for DS transmission.
2. The second one is to reserve only one dedicated HARQ process for SPS transmission. The two consecutive SPS data blocks are stored at the same HARQ buffer at the UE side. For the HARQ retransmission, the UE can know which data block is being retransmitted from the NEW DATA INDICATION (NDI) in the DL PDCCH which is shown in FIG. 3. For example, SPS data 1 and SPS data 2 are sent down consecutively and both need HARQ retransmissions. The UE keeps these two TBs in the same HARQ process (ID1 for example). Before SPS data 2 is sent down, the UE receives the retransmission with PDCCH indicating NDI=0. Then the UE knows that this NDI=0 indicates the SPS data 1 and NDI=1 indicates another SPS data. So there is no ambiguity from this point of view. However, the second one cannot work for some scenarios. For example, if the PDCCH for SPS data 1 retransmission with NDI=0 is missed by the UE or no SPS data 1 retransmission happens before the SPS data 2 transmission, the UE does not know NDI=0 is for SPS data 1. When the UE receives the SPS data 2 retransmission with NDI=1, the UE does not know whether this NDI=1 is for the SPS data 1 or SPS data 2. That means the ambiguity between two SPS data's retransmission cannot be resolved. This is shown in FIG. 4. The further drawback is that one dedicated HARQ process is reserved and OS cannot use it even it is free, which decreases the OS throughput as the first proposal does.
Therefore, one solution is needed which solves the SPS transmission ambiguity while improving DS throughput with respect to HARQ process for SPS transmission of UL.